1. Field of the Invention
The present invention concerns a method to automatically differentiate whether a pixel of a magnetic resonance image (acquired with a magnetic resonance system) originated from a fat-dominated tissue or water-dominated tissue, as well as a corresponding magnetic resonance system to differentiate whether a pixel of an MR image (acquired with the magnetic resonance system) originated from fat-dominated tissue or water-dominated tissue.
2. Description of the Prior Art
Imaging by magnetic resonance, i.e. magnetic resonance tomography (MR tomography) has ever-increasing fields of application in medical diagnostics. There are many applications of magnetic resonance tomography in which it is desired to differentiate tissue types that have a water-dominated content or fat-dominated content. For example, to determine damping maps for MR/PET systems it is necessary to segment tissue containing water and fat. Such a differentiation can also be usable in a combined MR and PET system (for example in a damping correction) or, for example, can be used in applications that have fat quantity determination or fat signal suppression.
The magnitude of the MR signal and the phase (i.e. the direction of the magnetization vector) that corresponds to the signal that exists for every pixel in an MR image. In conventional methods to differentiate water-dominated and fat-dominated tissues, it is typical to detect two (or more) magnetic resonance signals per pixel. In such methods (known as two-point Dixon techniques or the like, for example), for each pixel, one magnetic resonance signal is acquired in which the phase of the magnetic resonance signal of aqueous tissue exhibits the same phase as a magnetic resonance signal of fat tissue, and another magnetic resonance signal is acquired in which the phase of the magnetic resonance signal of the aqueous tissue is opposite the phase of the magnetic resonance signal of fat tissue. With the use of the two magnetic resonance signals per pixel, it is then possible to differentiate between water-dominated and fat-dominated tissue. However, this method requires a relatively large time duration since at least two spin echo-based magnetic resonance signals (one in phase and one in opposite phase) must be detected per pixel. The use of this technique is therefore not feasible in many cases. Additional approaches attempt to use a histogram-based fat/water separation together with imaging protocols that create significant differences in the signal intensities of fat-dominated and water-dominated tissues. However, these methods are not very stable, in particular if the entire body is examined and acquisition coils with inhomogeneous sensitivity are thereby used.